Suggest `len` instead of `count` on arrays, slices, or vectors

[teor2345]

Given the following code:

fn main() {
    let slice = [1,2,3,4];
    let vec = vec![1,2,3,4];
    
    println!("{} {} {}", slice.count(), vec.count(), vec.as_slice().count());
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8bd1115b12adc0eb5feb16bbb63a06cb

The current output is:

error[E0599]: the method `count` exists for array `[{integer}; 4]`, but its trait bounds were not satisfied
 --> src/main.rs:5:32
  |
5 |     println!("{} {} {}", slice.count(), vec.count(), vec.as_slice().count());
  |                                ^^^^^ method cannot be called on `[{integer}; 4]` due to unsatisfied trait bounds
  |
  = note: the following trait bounds were not satisfied:
          `[{integer}; 4]: Iterator`
          which is required by `&mut [{integer}; 4]: Iterator`
          `[{integer}]: Iterator`
          which is required by `&mut [{integer}]: Iterator`

(repeated 3 times with slight variations)

Ideally the output should:

• suggest using slice.len() instead of slice.count()

This is a common error, particularly for new users, and when switching between iterators and vectors/slices/arrays.